Around 12 inches works well for most vegetables and herbs, giving roots enough room while keeping material costs reasonable. Shallower beds (6–8 in) are fine for leafy greens and herbs, while root vegetables like carrots or parsnips benefit from 18 inches or more.

For one or two small beds, bagged soil is usually more convenient despite costing more per cubic yard. Once you’re filling several beds or a larger area, bulk delivery is typically far cheaper per yard and saves multiple store trips.

A layer of cardboard or landscape fabric at the very bottom helps block weeds, and a few inches of gravel or crushed stone above that keeps water moving instead of pooling around roots. Quality topsoil or garden mix then fills the rest, since most plant roots stay within the top 12 inches regardless of total bed depth.

A single 4×8 ft bed at 12 inches deep commonly runs somewhere in the low hundreds once you add frame materials, soil, and a few starter plants — though the exact number swings a lot based on frame material and whether you use premium or layered fill. Use the calculator above with your own dimensions and material choices for a number specific to your project.

Cool-season crops like lettuce, spinach, and radishes generally tolerate soil as cool as 40–50°F, while warm-season crops like tomatoes, peppers, and cucumbers need soil consistently above about 60–65°F before they’ll thrive outdoors. A basic soil thermometer gives a more precise read than air temperature alone.